1. Field of the Invention
The present invention relates to a method for preparing .alpha.-olefin polymers. More specifically, the present invention is to provide a method by which the production in a high yield of particulate polymers having good particle properties is made possible when it is applied to polymerization of an .alpha.-olefin, and, in particular, a method by which the production of particulate polymers that are free from the problem of adhesion of particles is made possible even when it is employed for producing polymers having a high rubber content.
2. Related Art
In recent years, highly active catalysts useful for the polymerization of olefins, which are a combination of a zirconocene compound and an alumoxane, have been proposed (Japanese Laid-Open Patent Publications Nos. 19309/1983 and 35007/1985). Further, it has also been proposed to design the structure of the ligand of the catalysts of the above type to make the preparation of various stereoregular polymers possible (Japanese Laid-Open Patent Publications Nos. 130314/1986, 295607/1988, 301704/1989 and 41303/1990). According to this proposal, polymers having high activity and a narrow molecular-weight distribution may be prepared. On the contrary, the catalysts are soluble in toluene used as a solvent, so that only a polymer whose particle diameter is extremely small (in general, approximately 1 to 50 .mu.m) may be obtained. For this reason, it has been difficult, to the best of our knowledge, by the above method, to industrially produce polymers with high efficiency. Moreover, it is necessary to use a large amount of an alumoxane in the above method, so that the production cost becomes high. In order to solve these problems, various proposals have been made. In Japanese Laid-Open Patent Publications Nos. 51408/1989, 275609/1989 and 140305/1991, alumoxane and a metallocene compound are brought into contact with each other in advance to obtain a particulate polymer. However, a particulate polymer can be obtained only when the above method is applied to so-called liquid phase bulk polymerization in which propylene is used as a solvent, and a polymer having a controlled particle diameter cannot be obtained when it is applied to slurry polymerization in which an inert solvent is used, or to gas phase polymerization.
In Japanese Laid-Open Patent Publications Nos. 296008/1986, 51407/1988 and 152608/1988, and WO 88-05058, such catalysts that a catalytic component is supported on an inorganic oxide, particularly on silica, are proposed. By this technique, it has become possible to prepare a particulate polymer even by gas phase polymerization. However, the activity per catalyst is low because the amount of the catalytic component which can be supported on silica is limited, and, in addition, the silica tends to remain in a polymer prepared. These are the causes of deterioration in the quality of the polymer, that is, the polymer is to have, for example, reduced moldability, a rough surface or fish eyes. This technique would therefore be unfavorable.
On the other hand, in Japanese Laid-Open Patent Publication No. 92621/1988 and WO 88-05058, it is proposed to support a catalytic component on polyethylene particles. By this method, deterioration in the quality of a polymer produced, which would be inherent in the use of an inorganic oxide as a carrier, can be prevented. However, a catalytic component would not be fully supported on polyethylene particles as compared with the case where an inorganic oxide is used as the support. For this reason, not only the activity becomes low but also the catalytic component is detached from the carrier while polymerization is conducted. As a result, a large amount of fine particles are produced, and thus the object which is to obtain a particulate polymer cannot be fully attained. Further, this proposal is carried out only for the preparation of ethylene polymers in which the activity is easily revealed. Therefore, a technique by which the preparation of particulate polypropylene is feasible has been desired.
When a propylene block copolymer having high rigidity and a high impact strength is prepared by polymerization, a rubbery copolymer is generally formed in an increased proportion in the resultant copolymer in order to enhance the impact strength of the resultant block copolymer. However, in this case, the amount of a by-product is increased, and polymer particles adhere to one another or to the inner wall of an apparatus. Therefore, it tends to be difficult to stably and continuously operate an apparatus used for preparing the polymer for a long period of time.
Recently, in order to improve the impact strength at low temperatures, proposals have been made such that, at a first stage, crystalline polypropylene is formed in the presence of liquid propylene and, at a latter stage, propylene and ethylene or an .alpha.-olefin having 4 to 20 carbon atoms are copolymerized, wherein use is made of a metallocene catalyst (EP-433989, EP-433990 and Japanese Laid-Open Patent Publication No. 114050/1992). As long as we know, however, although the impact resistance at low temperatures of the polymer product may be improved when the polymer is produced by the method proposed, a finely particulate polymer would be formed, and polymer particles would also tend to agglomerate with one another or to adhere to the inner wall of a polymerization vessel when the amount of the polymer produced at the latter stage is increased. Therefore, it would not be easy to prepare a copolymer stably by this method.
On the other hand, elastomeric polyolefins, which are useful as impact resistant polymer or as an additive for improving impact resistance of "hard" polymers, may be produced with catalysts based on vanadium used in a solution polymerization method. The vanadium based catalysts may have some drawbacks that it may not be feasible to conduct polymerization at elevated temperatures whereby productivity of polymers per a polymerization vessel may be at a lower level resulting in production of polymers at a higher cost.
The vapor/gas phase polymerization, which may produce polymers in higher productivity, has been desired.
The present invention has been made in order to solve the aforementioned problems in the prior art. Objects of the present invention are therefore to provide a catalyst for use in the polymerization of olefins, capable of producing polymers which have been improved in their particle properties even when the catalyst is used in slurry polymerization or gas phase polymerization, in particular, a catalyst capable of producing polypropylenes, of which production has been difficult by the prior art technology, without using a large amount of an alumoxane compound; and a method for preparing an olefin polymer by using the catalyst.
Another object of the present invention is to prepare propylene block copolymers and/or thermoplastic elastomers having improved impact resistance without entailing the formation of fine particles, agglomeration of particles or adhesion of particles to the wall of a polymerization vessel even when a rubbery copolymer is formed in an increased ratio.